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Thyroid dysfunction and diabetes mellitus (DM) are two common endocrine disorders that share overlapping metabolic pathways. While several studies have explored the association between thyroid function and DM, findings have been inconsistent. Specifically, both elevated and suppressed thyroid-stimulating hormone (TSH) have been variably reported as risk or protective factors for diabetes, and the contribution of thyroid autoimmunity (thyroid peroxides antibody, TPOAb) to diabetes risk remains particularly contested across study populations. This study aimed to evaluate the relationship between thyroid function markers-TSH, free thyroxine (free T4), and TPOAb - and the presence of diabetes in Korean adults. This cross-sectional study analyzed data from 4910 adults aged 19 years or older who participated in the Korea National Health and Nutrition Examination Survey (KNHANES VI) from 2013 to 2015. Serum levels of TSH, free T4, and TPOAb were measured, and DM was defined based on fasting glucose levels, use of antidiabetic medication, or self-reported diagnosis. As KNHANES does not classify diabetes by etiological subtype, the DM group predominantly reflects type 2 diabetes, consistent with its overwhelming predominance among Korean adults; type 1 diabetes could not be separately identified. Multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for diabetes across thyroid function categories, with subgroup analyses by age and sex. High TSH levels were significantly associated with an increased risk of diabetes (OR = 2.82; 95% CI: 1.26-6.32), especially in men under 65 years (OR = 5.10; 95% CI: 1.65-15.77). Low TSH levels were inversely associated with diabetes risk (OR = 0.35; 95% CI: 0.15-0.86). Low free T4 levels were also linked to an increased risk (OR = 2.08; 95% CI: 1.16-3.75), particularly in younger men. TPOAb positivity showed no significant overall association with diabetes, although a potential protective trend was observed in older men. Altered thyroid function, especially high TSH and low free T4 levels, is associated with increased diabetes risk in Korean adults. These findings suggest the clinical utility of thyroid function screening in diabetes risk assessment and highlight the need for further longitudinal research.